1. Field of the Invention
This invention relates to an air/fuel ratio detection system for a multicylinder internal combustion engine, more particularly to a system which can select one from among a plurality of outputs of an air/fuel ratio sensor sampled at a most optimum timing under the engine operating conditions even when the distances of the individual cylinder exhaust ports to the sensor are not equal for each cylinder and based on the sampled datum, to detect the air/fuel ratios of the individual cylinders correctly.
2. Description of the Prior Art
It is a common practice to install an air/fuel ratio sensor in the exhaust system of an internal combustion engine to detect the air/fuel ratio at that location. A system of this type is taught by Japanese Laid-Open Patent Application No. Sho 59(1984)-101,562, for example. Similarly, the assignee earlier proposed designing a model describing the behavior of the exhaust system detected by an air/fuel ratio sensor disposed at the exhaust confluence point, and designing an observer for estimating the air/fuel ratios at the individual cylinders based on the confluence point air/fuel ratio. (Japanese Laid-open Patent Application No. Hei 5(1993)-180,059 which was filed in the United States under the number of 07/997,769). Moreover, Japanese Laid-open Patent Application Hei 1(1989)-313,644 proposes a technique in which the appropriateness of air/fuel detection is checked at every predetermined crank angular position.
In the air/fuel ratio detection, since the remaining burned gas in a cylinder is swept out by the piston in the course of an exhaust stroke, the behavior of the air/fuel ratio at the exhaust system confluence point of a multicylinder internal combustion engine is conceived to be synchronous with the TDC (Top Dead Center) crank positions. When the air/fuel ratio sensor is installed at the exhaust system confluence point, it therefore becomes necessary to sample outputs of the sensor synchronized with the TDC crank positions. However, depending on the sampling timings, the control unit of the air/fuel detection system recognizes the air/fuel ratio as having a different value. Specifically, assume that the actual air/fuel ratio at the exhaust confluence point relative to the TDC crank position is that as illustrated in FIG. 26. As illustrated in FIG. 27, the air/fuel ratio sampled at inappropriate timings is recognized by the control unit as being quite different from that sampled at appropriate (best) timings. The sensor outputs should preferably be sampled at a timing which is able to reflect the change of the sensor output faithfully, in other words, the sensor outputs should preferably be sampled at a timing as close as possible to a turning point such as a peak of sensor outputs.
Further, the air/fuel ratio changes differently depending on the length of the arrival time at which the exhaust gas reaches the sensor, or depending on the reaction time of the sensor. The arrival time varies depending on the pressure and/or volume of the exhaust gas, etc. Furthermore, since, to sample sensor outputs synchronized with the TDC crank position means to conduct sampling on the basis of crank angular position, the sampling is not independent from engine speed. Thus, detection of the air/fuel ratio greatly depends on the operating conditions of the engine. For that reason, the aforesaid prior art system (1(1989)-313,644) discriminates at every predetermined crank angular position as to whether not the detection is appropriate. The prior art system is, however, complicated in structure and disadvantageous in that the discrimination becomes presumably impossible at a high engine speed since it requires a long calculation time. Further, there is the likelihood that, when a suitable detection timing is determined, the turning point of the sensor output will have already passed.
Furthermore, when the engine is a multicylinder internal combustion engine, the air/fuel ratio sensor is installed at, or downstream of, the confluence point of the exhaust manifold of the engine. Depending on the configuration of the exhaust manifold of the engine, it sometimes happens that the distances between the individual cylinder exhaust ports and the air/fuel ratio sensor are not the same for each cylinder or combination of cylinders. For example, when the engine is a V-type six-cylinder engine having two three-cylinder banks as will be explained with reference to FIG. 1, the respective cylinders do not always have equal distances from their exhaust ports to the air/fuel ratio sensor. As a result, the exhaust gas generated at a cylinder closer to the sensor arrives at the air/fuel ratio sensor at a time earlier than that generated at a less close cylinder, provided that the operating conditions of the engine remain unchanged.
It is therefore impossible to obtain a proper value when the sampled data selection is carried out paying attention only to the operating conditions of the engine, if the distance to the air/fuel ratio sensor is not uniform for all cylinders of the engine.
This invention is accomplished in view of the foregoing problems and has as its object to provide an air/fuel detection system for a multicylinder internal combustion engine which can select one from among the sampled outputs of an air/fuel ratio sensor that reflects faithfully the actual behavior of the air/fuel ratio at the exhaust confluence point and to detect or determine the air/fuel ratio of the engine even when the distances from the cylinder exhaust ports to the air/fuel ratio sensor are not equal and are different for some or all of the cylinders, thereby enhancing detection accuracy.
Another object of the invention is to provide an air/fuel ratio detection system for a multicylinder internal combustion engine which can select one from among sampled outputs consecutively generated by an air/fuel ratio sensor that reflects faithfully the actual behavior of the air/fuel ratio at the exhaust confluence point, and to determine the air/fuel ratio for the individual cylinders of the engine even when the distances from the cylinder exhaust ports to the air/fuel ratio sensor are not equal and are different for some or all of the cylinders, thereby making it possible to carry out cylinder-by-cylinder air/fuel ratio control for the engine.
Still another object of the invention is to provide an air/fuel ratio detection system for a multicylinder internal combustion engine which can select one from among sampled outputs consecutively generated by an air/fuel ratio sensor that reflects faithfully the actual behavior of the air/fuel ratio at the exhaust confluence point even when the distances from the cylinder exhaust ports to the air/fuel ratio sensor are not equal and are different for some or all of the cylinders and which is simple in structure.
For realizing these objects, the present invention provides a system for detecting air/fuel ratio of an internal combustion engine having a plurality of cylinders by sampling outputs of an air/fuel ratio sensor installed at a confluence point of an exhaust system of said engine, including engine operating condition detecting means for detecting operating condition of said engine, sampling means for sampling said outputs of said air/fuel ratio sensor, characteristic determining means for determining a characteristic for datum selection with respect to said operating condition of said engine, selecting means for selecting one from among said sampled data by retrieving said determined characteristic by said detected operating condition of said engine, and determining means for determining said air/fuel ratio of said engine based on said selected sampled datum. The characteristic features of the system is that said engine is provided with an exhaust manifold connected to said plurality of cylinders and having said confluence point where said air/fuel ratio sensor is installed in such a manner that distance from the air/fuel ratio sensor to the exhaust port of at least one cylinder in said group is different from that of the other cylinder, said characteristic determining means determines said characteristic for datum selection with respect to said operating condition of said engine and said distance to said air/fuel ratio sensor, and said selecting means selects one from among said sampled data by retrieving said determined characteristics by said detected operating condition of said engine and said distance to said air/fuel ratio sensor.